Hydroxy-functional acrylic resins have been widely and increasingly used in high performance coatings, particularly in automotive topcoats, due to their excellent durability and outstanding physical properties. They are usually crosslinked with a multifunctional isocyanate or a melamine to form acrylic-urethane or acrylic-melamine coatings.
Hydroxy-functional acrylic resins are usually copolymers of a hydroxyalkyl acrylate or methacrylate and one or more ordinary alkyl acrylates or methacrylates. Commonly used hydroxyalkyl acrylates and methacrylates include hydroxyethyl acrylate (HEA), hydroxypropyl acrylate (HPA), hydroxyethyl methacrylate (HEMA), and hydroxypropyl methacrylate (HPMA).
Usually, a combination of two types of ordinary alkyl acrylates and methacrylates are used to achieve optimal resin properties. The first type are acrylates and methacrylates whose homopolymers have low glass transition temperatures (T.sub.g below 25.degree. C.), e.g., n-butyl acrylate (T.sub.g : -54.degree. C.), n-butyl methacrylate (T.sub.g : 20.degree. C.), and 2-ethylhexyl methacrylate (T.sub.g : -10.degree. C.). The second type includes acrylates and methacrylates whose homopolymers have high T.sub.g s (greater than 50.degree. C.), such as methyl methacrylate (T.sub.g : 100.degree. C.). Styrene is also often incorporated into acrylic resins as a high T.sub.g monomer (T.sub.g : 99.degree. C.). High T.sub.g monomers increase the coating's gloss and hardness, while low T.sub.g monomers impart toughness and flexibility.
In general, high-solids acrylic resins have a hydroxyl number from 60 to 160 mg KOH/g, and a number average molecular weight (Mn) from 1,000 to 5,000. Lowering the molecular weight of the hydroxy-functional acrylic resin can reduce its solution viscosity. This is desireable because it reduces the amount of solvent required to make the coatings sprayable. Solvents are regulated as volatile organic compounds (VOCs) by the U.S. EPA and most coatings have VOC content limits imposed on them. However, the molecular weight reduction must be compensated by an increase in resin hydroxyl number to maintain the coating performance. The increased hydroxyl number increases hydrogen bonding within the resin which increases viscosity. The current solids level of sprayable acrylic-urethane or acrylic-melamine coatings is about 50% to 55% by weight.
Newly developed hydroxyl acrylic resins from allylic alcohols have significantly reduced viscosity, and their melamine and urethane coatings can achieve about 60% solids (see, e.g., U.S. Pat. No. 5,646,213). However, ultra-high-solids (65%-70%) acrylic-urethane and acrylic-melamine coatings are needed.